.box{
    width: 100%;
    height: 100%;
    display: flex;
    flex-direction: column;
}
header{
    width: 100%;
    height: 50px;
    display: flex;
    justify-content: center;
    align-items: center;
    background: dodgerblue;
    h1{
        color: #fff;
    }
}
main{
    overflow-y: scroll;
    flex: 1;
}